This invention relates to polyisocyanate-based foams, particularly to polyurethane or polyurethane-polyisocyanurate foams prepared using halocarbon blowing agents.
It is well known to prepare cellular materials by the reaction of polyisocyanates with active hydrogen-containing materials in the presence of a halocarbon blowing agent. In preparing flexible polyurethane foam, for example, halocarbons are often used to supplement water in generating a gas for blowing. However, the use of halocarbon blowing agents is particularly significant in making rigid foam. In rigid foam, the blowing agent provides thermal insulating properties as well as causes the foam to expand. For this reason, halocarbons usually constitute the major proportion of the blowing agent in rigid foam. Of the halocarbons, Refrigerants 11, 12 and 113 are of by far the most commercial importance, because of the combination of thermal insulating properties, boiling points and stability of these particular halocarbons.
Recently, the so-called "hard" halocarbons have been suspected of causing environmental damage in that it is thought the they might contribute to the destruction of atmospheric ozone. Accordingly, it is highly desirable to replace the hard halocarbons with other blowing agents which are not believed to cause environmental damage.
Unfortunately, the halocarbon blowing agents most commonly used in preparing polyisocyanate-based foam, and rigid foam in particular, are "hard" chlorofluorocarbons. "Hard" chlorofluorocarbons are those in which all hydrogen atoms of the corresponding alkane have been replaced with fluorine or chlorine. These halocarbons are particularly resistant to degradation in the environment, and therefore do not break down before reaching the atmospheric ozone layer, where they are believed to cause environmental damage. "Soft" halocarbons, which do contain one or more hydrogen atoms, more readily dissociate under conditions encountered in the atmosphere, and therefore less reaches the ozone layer of the atmosphere in a form which could cause significant damage.
Several "soft" CFCs exist which have boiling points similar to those of the most commonly used hard CFCs. However, the use of these "soft" CFCs has been limited due to their relatively high price and because the are not usually as efficient at blowing as the most common hard CFCs.
Several higher-boiling compounds are known which have very low conductivities, and would be considered as good candidates as blowing agents, except their use causes great dimensional instability in the foam. When the foam is at ambient temperatures, these high-boiling compounds condense, causing a partial vacuum in the cells, so the pressure of the surrounding atmosphere crushes the foam. Accordingly, these high-boiling CFCs have not been used in making polyurethane foam.
Because of the excellent thermal properties of certain of the high boiling CFCs, it would be desirable to take advantage thereof in preparing polymeric foam. Further, in view of the problems associated with the use of "hard" CFCs, it would be desirable to reduce or eliminate their use in making polyisocyanate-based foam.